How do I bill my clients and manage their billings?

Reseller Plan > New control panel
Most of our reseller uses Whmcs to manage their client billings.  You can find more info at http://www.whmcs.com/

In short, you can setup http://www.whmcs.com/ and connect it with your Dreamnix.com Reseller account.  Then you can do all the billings through this software.